For this domain, I have pointed the DNS records to FastMail, so stuff like SPF and DKIM are automatically set up on the FastMail side.

Of course, you have to set up aliases/identities etc in FastMail yourself - that's why I suggested that you take screencaps of all the current settings in Gandi, so you can check you're setting up all that you require.

As for, as I mentioned previously, still being able to send from your Gandi e-mail accounts, it is possible but not something I would do myself.

I only need to create a new identity from an external account (in this case Gandi) within Fastmail and I can send and receive with this Gandi email account, without needing to change records or upgrading my account.

For this domain, I have pointed the DNS records to FastMail, so stuff like SPF and DKIM are automatically set up on the FastMail side.

Of course, you have to set up aliases/identities etc in FastMail yourself - that's why I suggested that you take screencaps of all the current settings in Gandi, so you can check you're setting up all that you require.

As for, as I mentioned previously, still being able to send from your Gandi e-mail accounts, it is possible but not something I would do myself.